%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  Find the median M of a set S of integer numbers
%  (the median is the unique element M of S such that there are as 
%  many elements of S which are smaller than M as there are elements
%  of S which are larger tham M --- if S has an even number of 
%  elements, median/2 is assumed to be false
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

:-consult_lib.

median(S,M) :-
      (M in S &
       xsize({Z : Z in S & Z < M},N1) &
       xsize({Z : Z in S & Z > M},N2) &
       N1 = N2)!.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% Sample goal:
%
% {log}=> median({18,5,-3,7,1,7,11,2},M).
% M = 5